Construct a map that shows the "focus" of each pixel. A value of 0 in the
output corresponds to a sharp pixel, whilst higher values correspond to more
blurred pixels.
Algorithm based on: Yiwen Luo and Xiaoou Tang. 2008. Photo and Video Quality
Evaluation: Focusing on the Subject. In Proceedings of the 10th European
Conference on Computer Vision: Part III (ECCV '08), David Forsyth, Philip
Torr, and Andrew Zisserman (Eds.). Springer-Verlag, Berlin, Heidelberg,
386-399. DOI=10.1007/978-3-540-88690-7_29
http://dx.doi.org/10.1007/978-3-540-88690-7_29
Note that this is not scale invariant - you will get different results with
different sized images...